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Red Maple leaf toxicity

From: Diane

Dear Jessica, I read with dismay the letter about the mare that apparently fell ill and died as a result of red maple leaf poisoning. I had just returned from a workshop on toxic plants at the Equine Affaire in MA. In that workshop, I was relieved to learn about the quantities of plants that had to be consumed to reach toxic levels in the horse. According to the presenter, a horse would have to eat at least 3 pounds of red maple leaves to be adversely affected. That's a lot of leaves. This information is very different from the data that you provided. Can you tell me where to read more about this? My horses have access to fields with red maple trees in them. I worry about this but have never had a problem. The red maple is vary prevalent in our area and would be hard to eliminate. Do you know if there are any studies that clearly identify quantities associated with toxic effects from various plants?

Sincerely yours, Diane

Hi Diane - alas, this mare's case is not unique or even unusual. Your veterinarian can confirm that horses die every year from ingesting wilted leaves from Red Maple trees. In fact, if you attempt to classify trees and hedges in terms of their relative danger to horses, the top three on the list will be Yew, Oleander, and Red Maple (acer rubrum).

As for the quantity of wilted Red Maple leaves that will kill a horse, you are right, 3 lbs is often mentioned as a lethal amount, but it's a really bad idea to trust that, much less count on that as an absolute! The reason that many people say "3 pounds" is that many sources suggest that a fatal amount of wilted Red Maple leaves is approximately 3% of a horse's weight. In theory, this would mean that 3 lbs of those leaves would kill an "average" 1,000 lb horse. That's IN THEORY.

In practice, 1 1/2 lbs of wilted Red Maple leaves can be toxic OR FATAL to even a healthy 1,000 lb horse, even if it is given quick attention and diagnosis and excellent veterinary care including transfusions.

Is it possible that you may have mis-heard the presenter who addessed this subject? I know that most presentations at Expos are intended to be positive and cheerful as well as informative, but in this case, "adversely affect" is really not the most appropriate term for the consequences of ingesting a toxic or fatal amount of a substance that has no antidote...

As I'm sure you know, very few horses are "average". Many horses weigh a great deal less than 1,000 pounds, and a correspondingly smaller quantity of wilted Red Maple leaves could be fatal to those horses. Many horses are also at greater risk because they are in less-than-ideal health and condition, some from age, some from illness or bad conditions earlier in their lives. Knowledgeable equine vets will tell you that it's just not safe to make assumptions - or to keep horses where they can get at the wilted leaves of Red Maples. It may be hard to believe that just 2 pounds, or even 1 1/2 pounds of pretty leaves could kill a 1,000 pound horse, but it happens - and if a horse is smaller, old, or health-challenged, an even smaller amount could be not just toxic but fatal.

In the case of that recent letter to HORSE-SENSE, the horse in question was 17 (no longer young), an Arabian (quite likely to be in the 750-800 lb range) and was also a previous abuse/neglect case that had been rescued by her current owner. It's very likely that it took considerably less than 3 pounds of wilted Red Maple leaves to kill this mare. The source of the leaves was one tree, not very near her pasture, but even so, the quantity of leaves she ate was enough to cause her death. Estimating a "safe" amount of wilted Red Maple leaves for horses is rather like estimating a "safe" amount of dark chocolate or raisins for dogs - it's not a useful exercise! A particular amount of either dark chocolate or raisins may be enough to kill an "average" dog outright, but that does not mean that any amount up to that amount would somehow be "safe". Not only are dogs very different, but if a dog is older, weaker, or ill, or stressed or organ-damaged, it might take much less than the full, official, average "fatal" amount to kill the dog, and even a relatively small amount might make the dog very sick - there are such things as "non-fatal poisonings", and who wants to put an animal at risk? When the danger is so great and so well-known, there's just no reason to take any chances. It's better just to keep dogs away from dark chocolate and raisins, and it's better to keep horses away from Red Maples and other highly toxic plantings.

If you are on a trail ride and your horse reaches up and grabs a mouthful of fresh leaves from a Red Maple, there's no need to worry, because it's the wilted leaves that are toxic, not the green leaves. Even a mouthful of wilted leaves from a fallen branch shouldn't cause you to worry if it's spring or early summer, because at that time, the leaves will be less toxic than they will be later in the year. But if your horses live in pastures containing or surrounded by Red Maples, there's a good chance that those horses will eventually be in a position to eat quantities of wilted leaves, and that is a very serious risk. In fact, Red Maples in a horse's pasture are in the same category as barbed wire fencing around the pasture - it's not just a risk, it's a disaster waiting to happen, and it's not a question of IF, but WHEN.


I've had quite a few letters from people saying "You must be wrong, Red Maples aren't really toxic, I know because my horse's pasture is full of Red Maples and she loves to crunch up the wilted leaves in fall and she's never been sick for a minute. What's your reaction to this?" My reaction is simple: your pasture may be full of maple trees, and your mare may adore eating their wilted leaves, and I'm happy to know that she is in excellent health, but the very fact that she is ALIVE and HEALTHY tells me that the maple trees in your pasture are not RED MAPLES (acer rubrum). Talk to your vet, or any equine vet. Talk to a Master Gardener in your area, talk to the Extension Specialist, talk to a forester, talk to an arborist - you need to find out exactly what sort of maple trees you have in the pasture. I don't believe that they are RED MAPLES. There are many other maples, some of which show red leaves in autumn - it's not the fall colour that defines a Red Maple. So for any HORSE-SENSE readers who believe that your horses, or a friend's horses, are immune to the effects of wilted Red Maple leaves, PLEASE THINK AGAIN. (If there ARE some horses anywhere that are actually immune, there are veterinarians everywhere who would love to have an opportunity to study those horses!) It's far more likely that you or your friends may have misidentified the specific type of maple trees in the horses' pastures. I say again: Not ALL Maple trees are toxic to horses - only the leaves of RED MAPLES (acer rubrum) are toxic, and only when they are wilted. There are many, many other maple trees.

Trees add a great deal to the beauty of a property, and it's best to use caution when you plant them AND when you remove them. Horse owners are well advised to protect their horses by removing toxic trees from their pastures and gardens, but they should be entirely certain of the identity of any tree they intend to remove. Obviously you don't want to expose your horses to Red Maples, but identify your trees before you remove them. It would be a shame to rampage through the property destroying innocent sugar maples, for example.

Some veterinarians and extension agents have suggested that there are other maples that may present a danger to horses; notably a hybrid of red maple and silver maple, created to produce more dramatically-coloured foliage in autumn. If you are considering planting these trees in your pastures or elsewhere on your property, I would suggest that just in case these hybrids DO create a risk to your horses, you might consider planting sugar maples instead.

The bottom line when it comes to horses and Red Maples is this: They don't mix. Again, in the particular case to which you referred, there was only ONE Red Maple on the property, and it was up by the house, not next to (and certainly not IN) the pasture. But you know how strong, and how damaging, storm winds can be - and that's often the cause of broken branches and wilted leaves getting into pastures with horses who then eat the leaves out of hunger or curiosity. Horses are VERY curious - that's why even a lush pasture or generous amounts of good hay aren't necessarily a guarantee that the horses in a pasture will leave toxic plants alone.

When you purchase a property with the intention of keeping horses on it, you need to tour the property and identify the trees, hedges, and other plantings BEFORE you bring your horses home. When I bought my present farm, it came with an ancient yew hedge that had to be dug out and disposed of before I could allow any horses on the place. If there had been any Red Maples on the property, they would have been removed as well. I love trees and hedges, but horse owners have certain obligations to their animals, and some plantings, although extremely attractive, simply don't belong around horses.


When it comes to determining how much of any toxic plant will kill or badly damage a horse, all I can tell you is that it's better to be safe than sorry. It's possible to establish an "average" amount that will most likely be fatal to a healthy horse of a certain weight, but it's not always so easy to be sure of the effects of a lesser amount. In fact, it's difficult to be completely sure of anything when you're dealing with plant poisoning - the exact same amount of the same plant might be consumed by two horses grazing side by side, with very different results (e.g., an amount that might not kill a four-year-old horse might kill the twenty-four-year-old horse standing next to him). The amount that might kill an old or thin horse might not kill a young or heavier one.

In terms of how much of any given toxic plant a horse might eat and survive, there are just too many variables. The plants themselves present some of those variables - some plants are more dangerous in spring, some at other times of year. The amount of rainfall, condition of the soil, and the presence or absence of competing plants will all have an effect on the health (and toxicity) of a given plant. You probably know that stress on a horse can make that horse more vulnerable to a toxin; you might not realize that stress on a toxic plant can cause the plant to become more toxic. Some toxic plants kill very quickly, whilst others - notably those that cause liver and/or brain damage - can damage and weaken a horse over time (and can eventually kill the horse, or can make the horse more vulnerable to a relatively small quantity of another toxin, perhaps from a completely different plant).

I can't give you a precise formula that will keep your horses safe from toxic plants - no one can. All I can do is suggest some good resources to help you evaluate your property, your plantings, and your horses' degree of risk.


First, if you're near any major university, look for Extension office publications. Almost every Extension office and Horse Council across the country has a publication on the subject of horses and toxic plants; many have a specific handout on the subject of horses and Red Maples. You can probably find a lot of these online. Here's an example of a particularly good web page - short, clear, and very understandable to a layperson - by the Wisconsin State Horse Council:

Another useful online resource is Cornell University's Poisonous Plants Informational Database - here's a link to the page for horse owners: . It hasn't been updated in several years, and there are some broken or malfunctioning links, but overall it is still a useful site. Cornell has an ongoing Red Maple research project, so if you want cutting-edge information, you might also want to contact Cornell and find out how to get in touch with the researcher(s), probably in the area of Environmental Toxicology. SOMETHING in those wilted leaves is obviously able to oxidize erythrocytes in horses, but as far as I know, no one is quite certain what the toxic element is, although gallic acid and tannins appear to be likely suspects.

There's an extremely helpful and well-illustrated book that I've recommended many times:

The Horse Owner's Field Guide to Toxic Plants, by Sandra Burger.


The ASPCA Animal Poison Control Center is an excellent resource for animal poison-related emergencies, and is staffed 24 hours a day, 365 days a year. You or your local veterinarian can get diagnostic and treatment information via telephone (the number is 1-888-426-4435), and you can use your credit card to pay the $50 consultation fee. The laboratories used by the Center for analyzing toxins are right here at the University of Illinois at Urbana-Champaign, and they do a great job.

One last thought: the only sure way to keep your horse from eating toxic plants is to keep those plants completely out of your horse's reach.


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